Hello
Not sure if you are still facing this issue. If you are then you could be running into the limitation that is listed in this Microsoft SCOM documentation
If you are looking for a workaround you may want to use the Log File Monitoring Framework MP from NiCE
https://www.nice.us.com/log-file-monitoring-scom-nice-logfile-mp/
It is a free solution and is built based on SCOM Log File monitoring features with additional features that help avoid issues that are in the above Microsoft documentation.
Hope this helps.